April 30, 2013
Dew Drops Help Keep Cicada Wings Fresh And Clean
April Flowers for redOrbit.com - Your Universe Online
A spritz of dew drops is all the cicadas on the East Coast need to keep their wings fresh and clean as they emerge from their 17-year slumber.
Chuan-Hua Chen, Alfred M. Hunt Faculty Scholar and assistant professor of mechanical engineering and materials science at Duke's Pratt School of Engineering, led the team in demonstrating tiny particles such as pollen can be cleaned from cicada wings by a phenomenon he calls jumping droplets. As dew drop coalesce together, the merged drop jumps off water-repellant surfaces. Powered entirely by the surface energy initially stored in the dew drops, the jumping motion is automatic.
The engineers used a specially designed high-speed video imaging system to capture the jumping water droplets on a cicada wing, as well as the associated self-cleaning processes. The results of this analysis were published in a recent issue of Proceedings of the National Academy of Sciences (PNAS).
"The ability of water-repellant surfaces to self-clean has conventionally been attributed to rain droplets picking up dirt particles," Chen said in a statement. "For this conventional wisdom to work, rainfall must be present and the orientation has to be favorable for gravity to effectively remove the rain droplets. These limits severely restrict the practical use of self-cleaning superhydrophobic surfaces.
"We have found, however, that the self-propelled jumping motion of the dew drops is very effective in dislodging contaminating particles, regardless of the orientation," Chen said. "These new insights can help guide the development of man-made surfaces that are not dependent on any external forces and are therefore truly self-cleaning."
Typically only a few inches long, cicadas are flying insects whose most common species emerge on a yearly basis. Some species in the US arrive every 17 years, however. As nymphs, they dig out of the ground and then molt, shedding their skins to reveal their wings. The full grown cicadas take flight, spending the next four to six weeks flying around searching for a mate. They attract a mate with their distinctive songs, then lay eggs in the ground and die. Thus the cycle begins anew.
Rows and rows of tiny bumps or domes of various heights and widths characterize cicada wings. These bumps look like upside-down ice cream cones with the tips projecting upwards. On this type of surface, a water droplet only touches the points of the bumps, creating pockets of air underneath the droplet. Much like a puck in an air-hockey game, the droplet is kept aloft by this cushion of air.
"Most cicadas are unable to clean their own wings because of their short appendages," said Gregory Watson of James Cook University. "Furthermore, these insects commonly live in areas where there is little rain over an extended period of time. However, the areas are humid, which provides the tiny dew droplets needed to 'jump clean' their wings."
"These findings point to an alternative route to achieve self-cleaning which is fundamentally different from the conventional wisdom involving rolling or colliding droplets on a superhydrophobic surface," Chen said. "Self-cleaning surfaces using the jumping-drop mechanism can work at any orientation, which is a huge advantage for applications with unfavorable orientations with respect to gravity, such as mobile electronics and building roofs."